The role of DNA polymerase eta in DNA damage response and p53 activation. This application is proposed to address the signaling pathway of DNA polymerase eta (PolH) in DNA damage response and p53 activation and the effect of the signal pathway interaction between p53 and PolH on cell survival and death. In an effort to characterize the role of p53 in DNA damage response, we found that PolH can be induced by DNA damage in a p53-dependent manner. PolH is the product of the Xeroderma Pigmentosum (XP) gene. XP is an autosomal recessive disorder, and XP patients are prone to early onset of malignant skin cancers. Interestingly, we found that knockdown of PolH enhances cell survival by inhibiting DNA damage-induced apoptosis. We also found that DNA damage-induced activation of p53 is impaired in both PolH-knockdown and PolH-null cells, which can be rescued by a reconstituted PolH. Furthermore, we found that PolH modulates DNA damage response via the ATM- ChK2-p53 pathway. Finally, our recent preliminary studies showed that the stability of PolH protein is decreased upon DNA damage and PolH physically interacts with Mdm2 and Pirh2, both of which are a p53 target gene and an E3 ligase. Taken together, we hypothesize that PolH activity is regulated by multiple pathways and PolH has novel functions in DNA damage response and p53 activation. To further address this, the following three specific aims are proposed: (1) to determine whether and how PolH expression is regulated at basal and DNA damage conditions;(2) to determine the functional significance of the interaction between PolH and Mdm2 or Pirh2;and (3) to determine the role of PolH in DNA damage response and p53 activation. PUBLIC HEALTH RELEVANCE: It is well-known that loss of p53 tumor suppressor leads to genome instability and Xeroderma Pigmentosum (XP) patients often exhibit a high frequency of genome instability. In addition, the magnitude of genome instability is much higher in cells lacking both Xeroderma Pigmentosum (XP) and p53 genes than cells lacking either one individually. Interestingly, we found that PolH, the XPV gene product, is a novel p53 target gene and knockdown of PolH enhances cell survival by inhibiting DNA damage-induced apoptosis. Surprisingly, we found that activation of p53 following DNA damage is impaired in both PolH-knockdown and PolH-null cells, which can be rescued by reconstituted PolH. Given the fact that loss of PolH predisposes XP patients to early onset of multiple malignant skin cancers and that p53 is a tumor suppressor, further studies to address how PolH modulates p53 activation and DNA damage response are highly significant and warranted.